1. The Field of the Invention
The present invention relates to optoceramics, a process for their production and their uses. The invention also relates to refractive, transmissive or diffractive optical elements manufactured from these optoceramics as well as imaging optics. These optoceramics and optical elements are permeable to visible light and/or infrared radiation. The optoceramics consist of a crystal network, i.e. they constitute poly-crystalline material that is composed of a multitude of individual crystals.
2. Description of the Related Art
An optoceramic according to the present invention is highly transparent material that is essentially single phase, polycrystalline and based on an oxide or other chalcogenide. Optoceramics are a subdivision of ceramics. “Single phase” in this context means that more than 95% by weight of the material, preferably at least 97% by weight, more preferably at least 99% by weight and most preferably 99.5 to 99.9% by weight of the material, is present in the form of crystals of the desired composition (target composition). The individual crystals are arranged densely and have densities relative to their theoretical densities of at least 99%, preferably at least 99.9%, and more preferably at least 99.99%. Accordingly, the optoceramics are nearly free of pores.
The use in imaging optics refers to the use of the optoceramics according to the present invention in shapes that have curved surfaces at the entry and/or exit position of light, i.e. they preferably have lens shapes.
Optoceramics are distinguished from glass ceramics by the fact that glass ceramics comprise high proportions of an amorphous glass phase next to a crystal-line phase.
Similarly, a distinction between optoceramics and conventional ceramics are the high densities of optoceramics which cannot be achieved in conventional ceramics.
Neither glass ceramics nor conventional ceramics have the advantageous properties of optoceramics, such as refractive indexes, Abbe numbers, values for relative partial dispersion and particularly the advantageous high transparency for light in the visible and/or infrared spectral range.
The optoceramics according to the present invention are transparent enough to be suited for optical applications. Preferably, the optoceramics are transparent in the visible spectral range or in the infrared spectral range. Most preferably, they are transparent in the visible as well as in the infrared spectral range.
“Transparency in the visible spectral range” in the context of the present invention means an internal transmittance within a range of at least 200 nm width between 380 nm and 800 nm, for example in a range from 400 to 600 nm, in a range of from 450 to 750 nm or preferably in a range from 600 to 800 nm of above 70%, preferably above 80%, more preferably above 90%, and particularly preferably above 95% at a layer thickness of 2 mm, preferably even at a layer thickness of 3 mm, and especially preferably at a layer thickness of 5 mm.
The percentages of internal transmittance given above relate to the maximum internal transmittance that can theoretically be achieved with the material that the respective optoceramic consists of. The maximum internal transmittance that can theoretically be achieved with a certain material is determined by measuring the internal transmittance of a single crystal made of the same material. Consequently, the percentages of internal transmittance are indicative of the reflection and scattering losses at the grain boundaries in the polycrystalline material, while absorption and reflection at the phase boundaries between adjacent atmosphere and material are neglected.
“Transparency in the infrared spectral range” in the context of the present invention means an internal transmittance within a range of at least 1000 nm width between 800 nm and 5000 nm, for example in a range of from 1000 to 2000 nm, in a range of from 1500 to 2500 nm or preferably in a range of from 3000 to 4000 nm of above 70%, preferably of >80%, more preferably of >90%, particularly preferably of >95% at a layer thickness of 2 mm, preferably even at a layer thickness of 3 mm, particularly preferably at a layer thickness of 5 mm.
Ideally, the material exhibits a transmission (including reflection losses) of more than 20% at a layer thickness of 3 mm and in a wavelength range of more than 200 nm width between 5000 nm and 8000 nm.
The optical elements obtainable from the optoceramics as described herein are particularly suitable for use in imaging optics like for example objectives with reduced color aberrations, especially with approximately apochromatic imaging properties. The optical elements manufactured from optoceramics according to the present invention are usable within lens systems in association with lenses of glass and other ceramic lenses as well, especially also in digital cameras, in the field of microscopy, microlithography, optical data storage or other applications.
The main target in the development of imaging optics is sufficient optical quality while maintaining compact and preferably light construction of the optical system. Especially for applications in digital image capturing within electronic devices like for example digital cameras, objectives in mobile phones and the like, the optical imaging system must be very small and light. In other words, the total number of imaging lenses must be kept as low as possible.
In the area of microscopy nearly diffraction limited imaging optics are needed for the ocular as well as the objective.
For the sector of military defense transparent optical systems are needed, which preferably show high transmissions in the visible wavelength region (380 to 800 nm) as well as in the infrared up to 8000 nm, ideally up to 10000 nm. Furthermore these optical systems must be resistant to external attack like mechanical influence, like for example collision, temperature, change in temperature, pressure etc.
For many other technologies like for example digital projection and other display technologies highly transparent material is needed. But also in mainly monochromatic applications like optical storage technologies compact systems can be realized by application of material with high refractive index.
Today, development of imaging optics is limited by optical parameters of the available material. With the glass melting and molding techniques, which are available today, only such glass types can be produced with sufficient quality that are located in the Abbe diagram underneath a line that runs through the points Abbe number=80/refractive index=1.7 and Abbe number=10/refractive index=2.0. More precisely, glasses with refractive indices between about 1.9 and about 2.2 and Abbe numbers in the range of from about 30 to about 40 tend to be unstable so that it is very difficult to manufacture such glasses in considerable amounts and sufficient quality. Similarly, glasses with refractive indices of between about 1.8 and 2.1 and Abbe numbers in the range of from about 30 and 55 tend to be unstable.
Next to refractive index and Abbe number the relative partial dispersion is also important when choosing optical material. If it is intended to produce almost apochromatic optical systems, the combination of material with almost equal relative partial dispersions and big differences in Abbe number is necessary. If the partial dispersion Pg,F is plotted against Abbe number, most glasses lie on a line (“normal line”). Desirable are materials in which the combination of Abbe number and relative partial dispersion deviates from the normal line.
The definitions of refractive index nd, Abbe number vd and relative partial dispersion Pg,F are known to the person skilled in the art and can be understood by studying the relevant technical literature. The definitions can, for example, be found in “The Properties of Optical Glass”; Bach, Hans; Neuroth, Norbert (Hrsg.), Berlin (u.a.): Springer, 1995.—(Schott series on glass and glass ceramics: science, technology, and applications; 1), XVII, 410 p.-2., corr. Print., 1998, XVII, 414 S.
Today, the only available materials that are located above the line in the previously mentioned Abbe diagram are single crystals and polycrystalline material. The production of single crystals with known crystal growing techniques, however, is very expensive especially for high melting components, because of the very expensive breeding crucible material; further, this method is subject to limitations with respect to chemical compositions. Furthermore, crystals cannot be produced in a near-net-shape or near-net-format manner, resulting in significant post-processing effort.
R2Ti2O7 single crystals can have high refractive indices (see Shcherbakova et al., Russ. Chem. Rev. 48, 423 (1979)). The production of single crystals is, as indicated above, very expensive and does not render manufacture of larger optical elements possible.
The article K. N. Portnoi et al., Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, Vol. 6, No. 1, 91 (1970) does not contain any data or hints regarding the refractive indices of polycrystalline materials.
The crystals that are described in Malkin et al. Phys. Rev. B 70, 075112 (2004) are made from Yb2Ti2O7 and are obtainable via Floating Zone Methods in large individuals. The thicknesses are indicated as being up to 1.5 mm.
Although polycrystalline ceramics are obtainable in a wide range of compositions, they usually show insufficient optical quality, especially as far as homogeneity of refractive index and transmission is concerned. By now only few composition ranges and structure types are known that provide for transparent ceramics with sufficient optical quality.
For example the Japanese laid-open patent application JP 2000-203933 discloses the production of polycrystalline YAG by application of a certain sintering process. The position of YAG in the Abbe diagram or Pg,F-diagram (nd=1.83, Abbe number=52.8; Pg,F=0.558; delta Pg,F=0.0031), which is not exotic enough and does not suffice for most applications, is a disadvantage of YAG for passive linear optical applications. The YAG system is disadvantageous because, although chemical variability is high, the structure only accepts trivalent cations. The possibilities of variation (tuning) of optical properties which are, besides other factors, influenced by the UV band gap structure, are therefore not sufficient for many purposes.
In U.S. Pat. No. 6,908,872 a translucent ceramic material is described which utilizes barium oxide as obligatory component in the ceramic. The thus obtained ceramic shows perovskite structure and is para-electric. However, ceramics comprising such barium-containing phases with perovskite structure show insufficient optical imaging quality. This results from the tendency of many perovskites to form distorted ferro-electrical crystal structures and thus loose their optical isotropy. This inter alia leads to an undesired birefringence of the crystals from which the ceramic is made. Furthermore, transmission in the blue spectral region (wavelength around 380 nm) is insufficient.
Transparent ceramics of the composition La2Hf2O7 (LHO) are known from Ji et al., “Fabrication of Transparent La2Hf2O7 Ceramics from Combustion Synthesized Powders”, Mat. Res. Bull. 40 (3) 553-559 (2005). Therein powders of the target composition are used that have been obtained by combustion reactions. Only such ceramics are obtained that show transparencies of 70% at a sample thickness of <1 mm, which is too low for optical applications.
Ti4+-comprising, active La2Hf2O7 as transparent ceramic material for scintillator applications is known from Ji et al., “La2Hf2O7:Ti4+ ceramic scintillator for X-ray imaging” J. Mater. Res., Vol. 20 (3) 567-570 (2005) as well as from CN 1 587 196 A.
0.5 at %-5 at % Tb3+-doped active LHO as transparent ceramic material is described in Ji, Y M; Jiang, D Y; Shi, J L in “Preparation and spectroscopic properties of La2Hf2O7:Tb” (MATERIALS LETTERS, 59 (8-9): 868-871 April 2005). These active, i.e. light emitting, lanthanum compounds are not suitable for the desired application as passive optical elements.
DE 10 2006 045 072 A1 describes an optical element including a single phase optoceramic. However, the materials are of cubic structure of the ZrO2 type which is stabilized by Y2O3. Such crystal structures are different from a stable cubic pyrochlore or stable fluorite structure and suffer the corresponding disadvantages.
Klimin, et al, in “Physics of Solid State”, Vol. 47, No. 8, 2005, describes single crystal material and polycrystalline compounds, however does not address optical grade transparency of any polycrystalline material. Polycrystalline material is in the form of pressed powder that is slightly consolidated at quite low temperatures not higher than 1400° C. This procedure does not and cannot result in a material with sufficient optical properties.
WO 2007/060816 describes a translucent ceramic expressed by the general formula: AxByOw (herein the condition of 1.00≦x/y≦1.10 is satisfied and w is a positive number to maintain the electrically neutral condition) as the major component. The crystal system of this major component is the cubic crystal system, which comprises the pyrochlore type compound. Here an A2O3-rich composition is described which leads to a pre-powder after pre-sintering in a certain oxygen-rich atmosphere and then in a second sintering in O2-flow to A2B2O7 translucent ceramics. Although the optoceramics described in this reference have cubic pyrochlore structures they are rich in A2O3 component, which leads to optical deficiencies.
EP 1 992 599 describes a polycrystalline optoceramic comprising an oxide of the stoichiometry A2+xByDzE7, wherein 0≦x≦1; 0≦y≦2; 0≦z≦1.6 and 3x+4y+5z=8 and wherein A is at least one trivalent rare earth metal cation, preferably Y, Gd, Yb, Lu, La and Sc; B is at least one tetravalent cation, especially Ti, Zr, Hf, Sn and/or Ge; D is at least one pentavalent cation, especially Nb and/or Ta; and E is at least one anion that is essentially divalent. Here an A2O3-rich composition leading to A2B2O7 transparent ceramics after vacuum sintering is described. Although the optoceramics described in this reference have cubic pyrochlore structures they are rich in A2O3 component, which leads to optical deficiencies.